Ask The Van

Would it be possible (at least in theory) to heat a small building using a heat pump, but using no energy to run the pump? It seems unlikely, but why not? I am familiar with the laws of thermodynamics. I know we are not supposed to be able to get work from a non-spontaneous process, like a heat pump. Say you used a Stirling engine to start the process. You could easily get five or ten times more thermal energy out than the energy used. Then you could use some of that energy to run the pump leaving the rest to heat the building. What am I missing here? This is a question about physics theory, not about engineering.